Hydraulic presses are now used extensively for compression molding of large and comparatively thin plastic products. Mold halves are fixed to upper and lower platens. The upper platen may typically be displaced under very substantial driving forces to compress a molding charge. Very strict control of platen parallelism is required to produce a satisfactory product. The planar platen surfaces that actually engage the molds may be required to deviate from a parallel relationship by no more than 0.001-0.005 inches per foot. A basic understanding of press parallelism control may be obtained by reference to U.S. Pat. No. 4,076,780 which issued on Feb. 28, 1978 to Ditto.
Many old hydraulic presses have a stationary lower platen and a movable upper platen that is displaced by a large overhead hydraulic ram. The guiding structures used to direct movement of the upper platen are generally inadequate to maintain the strict platen parallelism required for contemporary compression molding. To adapt such presses for such strict parallelism control, hydraulic leveling cylinders may typically be mounted at each corner on the lower platen. These act against the upper platen and apply forces countervailing the action to the central ram. Alternatively, the leveling cylinders may be fixed to the upper platen and engageable with the lower platen. Each leveling cylinder may be associated with a platen spacing sensor that indicates the relative spacing of the platens proximate to the leveling cylinder. Such sensors may comprise, for example, a magnetic track that displaces with the upper platen and a complementary sensor head that is fixed to the stationary lower platen. In an arrangement described in U.S. Pat. No. 4,828,474 to Ballantyne, the platen spacing sensors are actually built into the leveling cylinders. A microprocessor will typically control the leveling cylinders in response to the spacing signals to maintain a common spacing between the platens adjacent to each cylinder and, assuming limited overall deformation of the platens, a parallel relationship. The common spacing value is changed as the press proceeds through preliminary compression and curing phases of operation.
A very significant problem in such prior presses appears to have gone unnoticed. The inventor has initiated finite element analysis to assess overall platen deformation in typical platens during compression phases of operation. Such analysis has revealed very significant local relative deflection of the platens in response to operation of the leveling cylinders themselves. The amount of such deflection can approach the alignment tolerances that must be maintained in parallelism control and varies with the force exerted by each cylinder. Since the spacing sensors identify such local relative deflection as part of the spacing between the platens, the spacing signals produced by the sensors do not reflect the actual alignment of the platens in the critical regions proximate to the mold. Instead of maintaining parallelism in such critical regions, the control system may actually induce a significantly non-parallel relationship at the mold itself. Such a problem may arise in more contemporary presses, depending on their exact configuration. The present invention addresses this problem.